This invention relates in general to processes of treating lignocellulosic materials and in particular to a process of producing a lignocellulosic pulp such as a wood pulp.
Lignocellulosic materials, such as wood, are plant materials made up primarily of cellulose, hemicellulose and lignin. The cellulose is the chief structural element and major constituent of the plants. It consists of a fibrous cellulose portion made from polymeric chains that are aligned with one another and form strong associated bonds with adjacent chains. The lignin is a three-dimensional polymeric material that bonds the cellulosic fibers and is also distributed within the fibers themselves. Lignin is largely responsible for the strength and rigidity of the plants. The hemicellulose is a polysaccharide that is a component of the cell walls of the plants. The two major classes of hemicellulose are glucomannans and xylan.
The wood is converted to pulp for use in paper manufacturing. Pulp comprises wood fibers capable of being slurried or suspended and then deposited on a screen to form a sheet of paper. There are two main types of pulping techniques: mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are digested with chemical solutions to solubilize portions of the lignin and hemicellulose and thus permit their removal in the waste pulping liquor. The commonly used chemical pulping processes include the kraft process, the sulfite process, and the soda process. The kraft process is the most commonly used and involves digesting the wood chips in an aqueous solution of sodium hydroxide and sodium sulfide. The wood pulp produced in the pulping process is usually separated into a fibrous mass and washed.
The wood pulp after the pulping process is dark colored because it contains residual lignin not removed during digestion which has been chemically modified in pulping to form chromophoric groups. In order to lighten the color of the pulp, so as to make it suitable for white paper manufacture, it is necessary to remove the residual lignin by use of delignifying materials and by chemically converting any residual lignin into colorless compounds by bleaching. Delignification and bleaching of wood pulp have been carried out with materials such as chlorine, oxygen or ozone.
U.S. Pat. No. 4,436,586 by Elmore discloses a method for producing both kraft pulp and alcohol from hardwood chips or the like. The chips are subjected to mild acid prehydrolysis following by mild caustic pre-extraction. The withdrawn hydrolysate has insufficient furfural to inhibit microorganism growth, and both the hexose and pentose sugars in the hydrolysate are fermented to ultimately produce ethanol, butanol, or the like. The chips, after caustic pre-extraction, are subjected to a sulphate cook, and a wash, and the resultant pulp is a kraft pulp said to have viscosity and tear strength characteristics more desirable than conventional kraft pulp. The pulp can be subjected to oxygen delignification, and it can achieve a higher K number in fewer subsequent bleaching stages than conventional kraft pulp.